The invention is directed generally to improvement in color television receivers, and particularly to an improved tint control for use in such receivers.
To reproduce color images, a color television receiver conventionally includes a 3.58 megahertz oscillator for developing a 3.58 megahertz sine wave of a controlled phase. That sine wave is used as an input to one or more demodulators to demodulate received color signals. The demodulated signals are then matrixed with received luminance signals, and applied to the control electrodes of a cathode ray tube.
To vary the hue or tint of a reproduced image, the receiver also includes a viewer-operable tint control. Frequently, the tint control is employed to vary the phase of the oscillator signal which is applied to the color demodulators. Such variation in the phase of the oscillator signal is typically effected by varying the phase of a 3.58 megahertz reference "burst" received as part of the composite television signal. The burst, in turn, is applied to a phase detector to generate a signal to control the phase of the oscillator in accordance with the phase of the burst.
Although such tint controls do provide control over the tint of reproduced color images, some of them suffer from the fact that they are A.C. controls as opposed to D.C. controls. That is, the viewer-operable knob by which tint is varied is coupled to internal receiver circuitry by leads which carry 3.58 megahertz signals. Consequently, those leads must be shielded, thereby adding expense to the receiver and some unwanted capacitance.
To avoid the problems associated with A.C. type tint controls, tint controls whose leads carry only a D.C. current have been developed. However, such D.C. tint controls have been more expensive to implement than is desirable.
Another shortcoming of some tint controls, both the A.C. and D.C. types, is the lack of a reproducible tint range. For example, one receiver may vary tint over a range of seventy degrees and another receiver of the same design may have a tint range of only fifty degrees. This lack of a reproducible tint range is, of course, undesirable. Thus, conventional tint controls of the A.C. and D.C. types have been less than prefectly satisfactory from a cost and/or reproductibility standpoint.